Shaping cycles with light: a regiodivergent approach to tetracyclic aza-aromatic compounds†

The development of regiodivergent methods that allow access to different structures from a single substrate through intramolecular processes is crucial for accelerating new molecule discovery, as well as making processes more sustainable and efficient in terms of waste production and economy. In this study, we report a novel regiodivergent cyclization procedure to access two distinct azapolyaromatic regioisomers from 2-alkynylazobenzenes. The key to achieving this regiodivergence lies in the presence or absence of a gold catalyst. The irradiation with visible light of 2-alkynylazobenzenes in the presence of a Ir photocatalyst affords 11H-indolo[1,2-b]indazoles, whereas under similar conditions with AuCl₃, indazolo[2,3-a]quinolines are produced. Control experiments and DFT calculations suggest that both transformations operate through different reaction mechanisms: the formation of 11H-indolo[1,2-b]indazoles involves a radical mechanism, whereas the formation of indazolo[2,3-a]quinolines appears to proceed predominantly through a polar mechanism. This transformation enables the one-step conversion of simple 2-alkynylazobenzenes into diverse azapolyaromatic structures via an intramolecular visible light-promoted process, holding significant potential for new nitrogenated heterocycles.